1. Field of the Invention
The present invention relates generally to encoded-sound-code decoding methods and sound data coding/decoding systems wherein ADPCM (Adaptive Differential Pulse Code Modulation) codes or APCM (Adaptive Pulse Code Modulation) codes are sequentially written to storage means, such as a ring memory, the method and system adapted such that, when codes thus written to the ring memory are to be reconstructed, the codes may be read out from optional addresses in the ring memory or from addresses starting from a next address to that at which the code writing to the ring memory has been suspended, thus providing continuous sound reproduction free from break-up of the sound.
2. Background Art
The ADPCM method has been known in the art as one of the voiceband data compression techniques. In this method, as to adjacent samples representative of sound data of time t.sub.n and t.sub.n+1, for example, a difference between a prediction value computed at time t.sub.n and the sound data of time t.sub.n+1 is found and encoded into a difference signal. In sound reproduction, an inversely quantized value of the difference signal is found by decoding the code, and the value thus found is added with the predictive value thereby to reconstruct a sound reproduction signal. According to the ADPCM method, a quantization step size necessary for inversely quantizing the difference signal is varied based on fluctuations of input signal level.
Some of the VTRs, tape recorders and the like have a function such as to sequentially write the sound reproduction data to the ring memory and to provide repetitive reconstructions of the sound data from the ring memory when a pause command is inputted.
In the prior art systems, the PCM (Pulse Code Modulation) codes rather than the ADPCM codes are written to the ring memory, enabling the sound reproduction from any of addresses thereof. Unfortunately, however, the PCM codes have lower compression efficiencies than the ADPCM codes, thus requiring a ring memory of a greater capacity.
Let us assume a case where the ADPCM codes are written to the ring memory for permitting the use of a ring memory smaller in capacity.
FIG. 1 schematically illustrates a configuration of a sound data coding/decoding system adapted such that ADPCM encoded sound data is temporarily stored in the ring memory and then the codes thus stored are reconstructed into sound-reproduction signals for output.
An input sound signal is encoded by ADPCM coding means 1. A code L.sub.n formed by the ADPCM coding means 1 is written to a ring memory 2. In sound reproduction, a code L.sub.n is read out from the ring memory 2 to be subject to ADPCM decoding means 3 for reconstruction into a sound-reproduction signal for output.
FIG. 2 schematically illustrates a configuration of the ADPCM coding means 1.
A first algebraic adder 41 finds a difference (prediction error signal d.sub.n) between an input signal x.sub.n to the ADPCM coding means 1 and a prediction signal y.sub.n by using the following equation (1) EQU d.sub.n =x.sub.n -y.sub.n (1)
An adaptive quantizer 42 forms the code L.sub.n by encoding (quantizing) the prediction error signal d.sub.n supplied from the first adder 41 based on a quantization step size .DELTA..sub.n. More specifically, the adaptive quantizer 42 finds the code L.sub.n based on the following equation and the resultant code L.sub.n is written to the ring memory 2: EQU L.sub.n =[d.sub.n /.DELTA.n] (2)
where the pair of signs "[ ]" are Gauss' notation for indication of a maximum integer within a range of not greater than a numerical value parenthesized therein.
A first quantization step size updating device 43 finds a quantization step size .DELTA..sub.n+1 for the subsequent sound signal sample .DELTA..sub.n+1 by using the following equation (3) where the code L.sub.n and a function M(L.sub.n) are in a predetermined relation: EQU .DELTA..sub.n+1 =.DELTA..sub.n .times.M(L.sub.n) (3)
A first inverse adaptive quantizer 44 uses the code L.sub.n for decoding (inversely quantizing) the prediction error signal d.sub.n thereby to find an inversely quantized value q.sub.n. More specifically, the first inverse adaptive quantizer 44 finds the inversely quantized value q.sub.n from the following equation (4): EQU q.sub.n =(L.sub.n +0.5).times..DELTA..sub.n (4)
A second algebraic adder 45 finds a sound-reproduction signal w.sub.n based on the prediction signal y.sub.n for the present sound-signal sample x.sub.n and the inversely quantized value q.sub.n. More specifically, the second adder 45 finds the sound-reproduction signal w.sub.n from the following equation (5): EQU w.sub.n =y.sub.n +q.sub.n (5)
A first predictor 46 finds a prediction signal y.sub.n+1 for the subsequent sound-signal sample x.sub.n+1 by delaying the sound-reproduction signal w.sub.n by one sampling time.
FIG. 3 schematically illustrates a configuration of the ADPCM decoding means 3.
A second inverse adaptive quantizer 51 assigns a code L.sub.n ' supplied from the ring memory 2 and a quantization step size .DELTA..sub.n ' supplied from a second quantization step size updating device 52 to the following equation (6), thereby finding an inversely quantized value q.sub.n ': EQU q.sub.n '=(L.sub.n '+0.5).times..DELTA..sub.n ' (6)
If the code L.sub.n ' found by the ADPCM coding means 1 is correctly transmitted to the ADPCM decoding means 3 or L.sub.n =L.sub.n ', the values q.sub.n ', y.sub.n ', .DELTA..sub.n ' and w.sub.n ' used by the ADPCM decoding means 3 are respectively equivalent to the values q.sub.n, y.sub.n, .DELTA..sub.n, and w.sub.n used by the ADPCM coding means 1.
The second quantization step size updating device 52 uses a code L.sub.n ' supplied from the ring memory 2 for finding a quantization step size .DELTA..sub.n+1 ' for the subsequent code L.sub.n+1 ' from the following equation (7), where the code L.sub.n ' and a function M(L.sub.n ') has the same relation as the code L.sub.n and the function M(L.sub.n) does: EQU .DELTA..sub.n+1 '=.DELTA..sub.n '.times.M(L.sub.n ') (7)
A third algebraic adder 53 finds a sound-reproduction signal w.sub.n ' based on a pre y.sub.n ' supplied from a second predictor 54 and the inversely quantized value q.sub.n '. That is, the third adder 53 finds the sound-reproduction signal w.sub.n ' from the following equation (8): EQU w.sub.n '=y.sub.n '+q.sub.n ' (8)
The resultant sound-reproduction signal w.sub.n ' is outputted from the ADPCM decoding means 3.
The second predictor 54 delays the sound-reproduction signal w.sub.n ' by one sampling time for finding the subsequent prediction signal y.sub.n+1 ' which is supplied to the third adder 53.